Modern wireless communication systems or networks, such as GSM (Global System for Mobile Communications) and IRIDIUM, determine an Individual Subscriber Unit (ISU) location and use that information for billing and accessing purposes. Knowledge of the location of the ISU allows the network to route ring-alert page messages, incoming calls, and other communications to the ISU. In addition, this location information allows the network to conform to diverse rules and procedures which may be imposed by political entities within whose jurisdiction the network may operate. Typically, an ISU is located on a map of a coverage area of the communication system. An explanation of location references and conventional techniques used to map the coverage area is presented below.
A typical technique for mapping a coverage area subdivides each Gateway (GW) or mobile telephone switching office (MTSO) area into Service Control Areas (SCAs) or billing territories which are further subdivided into Location Area Codes or LACs. The SCA or billing territory represents a region throughout which access rules and billing rules are consistent. Each SCA or billing territory is wholly contained within a single GW or MTSO area and is a subdivision of that GW area. The LAC represents a smaller region, completely contained within the SCA. This smaller division is utilized by the system as a system location reference for an ISU and is used by the system to label the location of the ISU.
This SCA/LAC subdivision of the system coverage area map is represented in a database form, and distributed to each GW area for use. A conventional SCA/LAC database contains data to effectively tile the entire system coverage area with tiles of varying sizes such that each tile is identified by its GW/SCAILAC affiliation. Boundaries defining SCA/LAC subdivisions are typically defined politically and geographically, not mathematically. Accordingly, generation, distribution and maintenance of the database defining these political and geographic boundaries is a substantial task. For the IRIDIUM system, each GW area has an Earth Terminal Controller (ETC) that stores this GW/SCA/LAC database which is unique for each GW area. GSM systems have a similar controller, but the LAC areas are defined by permanent cell site locations and need not be mapped. Simple cell site identities serve as system locations for ISUs. Moreover, when a particular GW area has system problems, transferring this large GW/SCA/LAC database to another GW area, to serve in a backup role, is a large task that takes a substantial amount of system overhead to complete, causing the system to suffer an extensive down time.
To locate an ISU, in the IRIDIUM system for example, a high resolution location known as a grid code or point location is determined. This actual location of the ISU or single-point grid code is compared to the GW/SCA/LAC database to determine the LAC in which the user is currently located. While this high resolution grid code is returned to the ISU, only the compromised LAC location is stored within a Visiting Location Register (VLR) to represent the users location. This LAC location is considered compromised in that it identifies a geographic area or LAC in which the ISU is located in contrast to the available high resolution grid code that gives a specific point location for the ISU. The VLR is a database that is used by a cellular switching office of that GW area to temporarily store data for the subscriber while the subscriber visits an area serviced by that VLR. The VLR provides subscriber location information to a Home Location Register (HLR) for routing calls to the subscriber. The HLR is a database, that is also used by the switching office, in which data on the subscriber is permanently stored. The compromised data stored in the VLR is of little use for subsequent processing and is not suitable for system performance/usage modeling that could be used to perform resource allocation.
Additionally, this compromised location information results in an inefficient use of ring-alert resources due to a need to ring the entire LAC area or several LAC areas rather than a more confined region. Broadcasting each ring-alert page message within a particular LAC requires a certain amount of energy. In terrestrial wireless (e.g., cellular) systems, there is less need to conserve energy because the energy supply is virtually unlimited. However, energy limitations are not the only concerns, as terrestrial wireless systems do have capacity limitations. In a satellite communication system, where subscriber units receive ring-alerts from satellite communication nodes, it is desirable to conserve both energy and capacity by only broadcasting ring-alerts in as few LACs as possible, since a satellite's energy supply is limited to that supplied by its solar panels and its batteries. Further, system capacity will limit broadcasting opportunities in both satellite and terrestrial systems. Energy expended and opportunities consumed for ring-alerts reduces the amount of energy and capacity available for other communications.
Accordingly, a need exits for a method of defining an ISU location in a wireless communication system that provides high resolution location data while minimizing the database used to map the coverage area therein.